The term "most vestigial" implies something that is the most rudimentary or unusable part of a system or an organism. The antonyms of this term will be words that describe something essential, important or functional. Some possible antonyms for "most vestigial" include "crucial," "significant," "important," "vital," "useful," "valuable," "relevant," "critical," and "indispensable." These words provide a stark contrast to the concept of "most vestigial" as they represent elements that are necessary for survival, growth, and development. Using these antonyms in context can help clarify the importance of certain features or components of a system.